The All About Birds bird guide and most field guides include range maps for each species. These maps use a color-code system to indicate the range and sometimes migration pathways for the selected species.
Range maps should be used with the understanding that the information provided is general in nature and is not intended to reflect exact limits of range. The accuracy of range maps is limited by several factors:
Some species expand their ranges over time, sometimes very quickly. The Eurasian Collared-Dove has spread from Florida to the U. S. west coast in just a few years.
As noted on the irruption page, some species may remain relatively far north in some years and suddenly expand their winter range the next. In non-irruption years a particular species can be totally absent from a state, only to become quite common in an irruption year.
Birds (especially young birds) often wander from the traditional migration path and end up in the "wrong" location.
The migration paths for some species are still not well known.
|The color code shown on the range map of the Black-throated Green Warbler shows three of the four typical location zones.
The brown color indicates where the bird can be found in the summer, which is its typical breeding range.
The yellow indicates areas through which the species normally migrates. In this case, the warbler migrates through much of eastern U.S. and into Mexico.
The blue area indicates the wintering range of the species.
As this map illustrates, some Black-throated Green Warblers continue to migrate through the southern part of Central America and into northern South America.
|This range map for Blue Jays is deceptively simple and does not reveal the complexity of Blue Jay migration. The map indicates a fairly static year round range for the species, with a slight extension to the southwest in the winter.
Hidden in the solid purple area of the year-round range are the migration paths of large numbers of Blue Jays. Some Blue Jays migrate while others do not. In some cases it appears that one population of Blue Jays may replace another. For example, Blue Jays that summer in Illinois might move further south during the winter and could be replaced by Blue Jays from Canada.
As described in the Bird Guide, the details of Blue Jay migration are not well understood.
|The Eastern Bluebird map shows both a summer and a winter range, indicating that a yellow migration color should be somewhere on the map. In this case, however, the migration route includes areas where birds are year round residents.
By studying the map, one could guess that the Gulf Coast states' winter bluebird population is larger than the summer population.
|The Cattle Egret map provides a good example of the complexity of illustrating seasonal changes in bird populations.
In this case the map shows a "winter (non-breeding)" range in blue that covers a large portion of the United States. After breeding, Cattle Egrets disperse throughout the U.S. and into Canada, resulting in the range shown. Confusion is created by combining the winter and non-breeding ranges. The egrets that spread throughout the U.S. after nesting eventually migrate south into limited winter ranges.
When studying range maps it is a good idea to read any accompanying text. For the Cattle Egret the species description in the Bird Guide section includes the following:
Winters in southern California, coastal Texas, Florida, and southward.
Understanding the range map of a species is often dependent on an understanding of the bird's complete migratorial behavior.
Range maps are a useful tool in determining if a certain bird can be found in your area at a particular time of year, just be cognizant of their limitations.
One of the best ways to obtain current information on which birds are being seen where is to visit Cornell's eBird web site. By using eBird, birders from all over the country can report sightings. The data collected provides a current look at where and when different species are being seen.